The present invention relates to improvements in the conversion of carbon monoxide into hydrogen and CO.sub.2 in the presence of steam and suitable catalysts.
It is known that the conversion yield (in practice the regulation of the content of CO issuing from the conversion apparatus) is greatly influenced, other conditions being left unchanged, by the amount of steam present in the reaction mixture.
To increase the said amount of steam in the gas entering the conversion apparatus, numerous systems and expedients have been proposed and used in the past, in which the excess steam or the heat which is contained in the gas issuing from the conversion apparatus, are recovered and recycled to the conversion to increase its yield.
The most simple of these methods consists in contacting the gas issuing from the conversion and rich in steam with a stream of water, which is thus heated. The stream of water thus heated is circulated and brought into direct contact with the gas to be delivered to the conversion apparatus, which gas is enriched in steam and thus saturated.
However, this method requires, as is known, a noticeable difference in temperature between the gases at the outlet and the inlet of the conversion apparatus, in order to allow transfer of the heat and thus of the steam.
In the present invention, as will be specified in the following, it is, on the contrary, suggested to carry out the said transfer of heat from the outlet to the inlet of the coversion with relatively small differences in temperature, even of the order of 10.degree.-30.degree. C.
It is also known to those skilled in the art that recently two temperature conversion methods have been utilized; the second stage, at low temperatures, which is usually carried out in the presence of a catalyst based on zinc or copper, is characterized in that the temperature of the gas at the inlet of the conversion is 220.degree. C., whereas the temperature of the gas issuing from the conversion is only 230.degree.-240.degree. C.
Under these conditions the transfer of heat and thus of steam from the gas issuing from the conversion apparatus to the gas entering the conversion apparatus can no longer be carried out by means of the aforesaid method.
An object of the present invention is thus to carry out the transfer of heat from the outlet to the inlet of the conversion even in the case of the above conversion method and in other similar cases in which the difference in temperature, as noted in the foregoing, is of the order of 10.degree.-30.degree. C. This will be explained in detail in the following.
Prior to the conversion, the gas is submitted to a special quench, called "saturation quench", different from the quench effected heretofore in the known art.
In the known art, the sensitive heat of the gas, down to 220.degree. C. or similar temperatures, was converted, in the presence of water, into steam which enriched thus the gaseous mixture.
In the present invention, on the contrary, the quench is carried out in a different manner and to a deeper degree, in the sense that the sensitive heat of the gaseous mixture is converted into steam until the said mixture cools down to the saturation temperature (which explains the term "saturation quench"), which in general in the usual installations corresponds to a value of 180.degree.-200.degree. C. As a result, the temperature obtained is sufficiently low to allow the heat present in the gaseous mixture after the conversion to be recovered and delivered to the gas fed to the conversion.
Generally, to this end, the gas issuing from the conversion, (at a temperature of 230.degree.-240.degree. C. or similar) is used to heat a stream of water up to a temperature generally of the order of 190.degree.-215.degree. C. The water thus heated is brought into direct contact with the gas before the conversion in accordance with the saturation quench method indicated above, yielding to the latter its heat with great ease. The water issuing from the saturated quench is then recovered and delivered to an exchanger in which it is heated by means of the heat of the gas issuing from the conversion.
According to the present invention it is also suggested that the gas issuing from the saturation quench be suitably heated up to 220.degree. C. or similar temperatures, in order to be delivered to the conversion apparatus at a temperature sufficiently high to prime the reaction. This heating may be carried out by using a number of embodiments, but in the description of the present invention two embodiments particularly easy and important are stressed, namely:
(1) The gas issuing from the conversion, at a temperature of about 300.degree.-350.degree. C. (i.e. the conversion gas which has previously yeilded heat to a boiler for the production of steam) is divided into two fractions; the first fraction possibly yields part of its heat to the water feeding the boiler and is then submitted to the saturation quench described above down to a temperature of 180.degree.-200.degree. C. At this point, it is mixed with the second fraction of gaseous mixture, the amount of which is suitably regulated to obtain a mixture with a temperature of 220.degree. C.
(2) The gaseous mixture coming from the conversion at high temperature, after its heat has been used for the production of steam and, in suitable conditions, also for heating the boiler feed water, is submitted to the aforesaid saturation quench down to a temperature of 180.degree.-200.degree. C. At this point it is heated up to 220.degree. C. or similar temperatures by means of a heat exchange with the gas at the outlet of the conversion.